The invention relates generally to current sensitive trigger circuits and in particular, is directed to circuits that are responsive to telephone supervisory and control signals such as dial pulses.
In a loop closure seizure step-by-step telephone system, each subscriber telephone set is connected to a central station by tip and ring conductors. The subscriber telephone set includes a set of normally open contacts that are closed when the receiver is lifted from the telephone set. Equipment at the central station accepts this loop closure as a seizure. Thereafter dialing of the telephone set will open and close the contacts in the telephone set a predetermined number of times for each number dialed by the subscriber. This intermittent opening and closing of the loop is used to actuate a stepping switch for interconnecting a first subscriber telephone set to any one of a number of second subscriber telephone sets pursuant to the number dialed by the first subscriber.
The central station includes a selector circuit that establishes a dial tone and selects the next idle path to whatever code is dialed by the subscriber. The dial tone is generated by an AC signal impressed upon either the tip or ring side of the conductors extending to the subscriber telephone set. The selector also impresses a DC voltage across the tip and ring conductors extending to the telephone set to create dial pulses when the telephone set is dialed. The selector further includes switching or pulsing circuitry for sensing and amplifying the dial pulses to a level suitable for actuation of a stepping switch that is used to interconnect one subscriber's telephone set to another subscriber's set according to the code dialed by one of the subscribers. In the prior art these pulsing circuits normally comprise an electrical relay having a set of break/make contacts. This relay serves to alternately energize the actuating coil of the stepping switch and an enabling relay for enabling operation of the stepping switch. Two actuating coils are provided on each pulsing relay. One end of both coils extends to the subscriber's telephone set via the tip and ring conductors while the other two ends of the coils are connected to a source of DC power and ground, respectively.
The mechanical contacts of this pulsing relay deteriorate through wear, pitting and/or corroding and become a relatively high maintenance item compared to the rest of the telephone system. In recent years there has been an increased emphasis on the use of more sensitive and faster responding control devices in telephone systems to provide improved service. For example, lower inertia and faster responding pulsing relays are presently being applied to circuits for control by telephone supervisory and control signals. However, such sensitive control devices under various conditions tend to produce distorted output pulses that at times exceed acceptable limits and tend to produce undesirable spurious additional contact closures due to bounce or flutter of the pulsing relay contacts.
Furthermore, these pulsing relays are normally connected to any one of a large number of subscriber telephone lines and each telephone line presents a different set of electrical characteristics depending upon its length, gauge, leakage, the type of telephone set it is connected to, etc. Each telephone line therefore presents a different value of capacitance, inductance and resistance between the subscriber's dial and the associated selector such that the operating conditions of the selector change with each telephone line connected thereto. Furthermore, due to the high cost of copper, there is a general tendency to use smaller diameter conductors to reduce costs. Telephone lines using these smaller conductors exhibit a higher resistance per unit length and may at times exhibit higher capacitance per unit length. Thus, there is a substantial need for increasingly sensitive selector circuits which facilitate the use of longer and thinner tip and ring conductors having widely varying electrical characteristics.